Closure devices are used to seal or close an opening, which is in communication with pressure or vacuum in pressure vessels. Some examples of these closures are disclosed in U.S. Pat. Nos. 835,645; 1,260,299; 2,822,109; 3,386,612; 3,419,180; 3,543,801; and 3,764,037. These prior devices have not been easy to use, particularly to open and then reseal an opening which has been closed for a long period of time. The closure assembly disclosed herein includes an improved retaining structure to secure the closure device in position.